A duct body having a substantially quadrilateral cross-sectional shape in section is provided at an opening end thereof with a flange extending along the opening edge for connecting the duct to another duct or the like. Specifically, a common-plate flange is formed by bending an end portion of each of steel plates constituting a duct body, then the surfaces of two such flanges are joined together and those flanges are held by means of flange-holding metal fittings (clips). In the corners of the flanges, corner pieces are provided, and the flanges are fastened there with bolts, thereby connecting the two ducts and the like (e.g., refer to Non-Patent Document 1).
There is, for example, a smoke-extraction duct for extracting smoke at the outbreak of a fire. This duct takes in air having a higher pressure than an ordinary duct does (e.g., threefold). Hence, the smoke-extraction duct requires a flange having a higher strength and rigidity, and the flange is formed by an angle material. Specifically, the flange is formed by connecting an angle material (angle steel) having a substantially L-shape in section to an opening end portion of a duct body by means of riveting or welding. The flange is formed with bolt insertion holes at predetermined intervals. Two such flanges are fastened by joining the surfaces of those flanges together and inserting bolts into the bolt insertion holes, thereby making the connection of the two ducts or the like (e.g., refer to Non-Patent Document 2). Besides, the angle material forming the flange is subjected to a coating, thereby inhibiting corrosion of the angle material.